In general, surfaces of components present in a lithographic projection apparatus become contaminated during use, even if most of the apparatus is operated in vacuum. In particular, the contamination of optical components in a lithographic projection apparatus, such as mirrors, has an adverse effect on the performance of the apparatus, because such contamination affects the optical properties of the optical components.
It is known that contamination of optical components of a lithographic projection apparatus can be reduced by purging a space of the lithographic projection apparatus in which such a component is located with an ultra high purity gas, referred to as a purge gas. The purge gas prevents contamination of the surface, for example, molecular contamination with hydrocarbons.
A drawback of this method is that the purge gas may have an adverse effect on the activity of chemicals used in the lithographic projection process. Thus, there is a need for a modified purge gas that does not adversely affect the activity of chemicals used in lithographic processes.